ATPASES AND PHOSPHATE EXCHANGE ACTIVITIES IN MAGNESIUM CHELATASE SUBUNITS OF RHODOBACTER-SPHAEROIDES

Three separate proteins, BchD, BchH, and BchI, together with ATP, inse rt magnesium into protoporphyrin IX. An analysis of ATP utilization by the subunits revealed the following: BchH catalyzed ATP hydrolysis at the rate of 0.9 nmol per min per mg of protein, BchI and BchD, tested individually, had no ATPase activity but, when combined, hydrolyzed A TP at the rate of 117.9 nmol/min per mg of protein, Magnesium ions wer e required for the ATPase activities of both BchH and BchI+D, and thes e activities were inhibited 50% by 2 mM o-phenanthroline. BchI additio nally catalyzed a phosphate exchange reaction from ATP and ADP, We con clude that ATP hydrolysis by BchI+D is required for an activation step in the magnesium chelatase reaction, whereas ATPase activity of BchH and the phosphate exchange activity of BchI participate in subsequent reactions leading to the insertion of Mg2+ into protoporphyrin IX.